The core of an optical cable is conventionally constituted by a stainless steel tube containing a bundle of optical fibers disposed loosely in the cavity of the tube. The tube is covered with a thick sheath of polyethylene. The tube provides mechanical protection for the bundle of fibers. In some cases it also serves as an electrical conductor for powering transmission equipment underwater, e.g. repeaters.
Such a cable must be proof against transverse ingress of water under very high pressure, and in addition it must also resist longitudinal propagation of water should ingress of water occur because of damage to the cable. Resistance to longitudinal propagation of water is conventionally obtained by filling the tube with a viscous hydrophobic material. However, such resistance is effective only after a certain length of intact cable. Where a cable accesses a repeater, there is not always a sufficient length of intact cable to guarantee sealing. In order to prevent water propagating inside a repeater, French patent application No. 2 595 149 teaches placing a longitudinal sealing device in the vicinity of each of the cable accesses to a repeater. Each repeater is provided with two short lead-in cables, each of which is terminated by a splice box for connection to a long cable. A longitudinal sealing device is inerted in each lead-in cable dividing it into two lengths. The prior sealing device comprises:
a block of plastic material molded mold over the end of the length of tube situated adjacent to the repeater, and over the optical fibers occupying a portion of the bundle situated between the two lengths of the tube; with the end of the length of tube situated adjacent to the repeater including a neck extending all around the tube for holding the block of plastic material on the end of the tube;
a metal sleeve surrounding the block of plastic material and brazed to the end of the length of tube situated adjacent to the splice box;
a metal plug threaded over the length of tube situated adjacent to the repeater so that the end of the tube projects beyond the metal plug to carry the block of plastic material; with the tube being brazed to the plug at its end furthest from the block of plastic material;
a clamping ring threaded over the sleeve for clamping the ring onto the metal plug, which plug has an outside thread; with final fixing of the sleeve and the plug (and also electrical continuity, where required) being ensured by weld spots; and
a hydrophobic substance injected into the space that remains between the sleeve and the block of plastic material.
The neck around the tube has the function of engaging the block of plastic material molded onto the end of the tube in such a manner as to obtain durable sealing in spite of possible interstices which may be in engendered along the inside wall of the tube by the shrinkage to which the molded plastic material is subject. This device works well when the relative transverse dimensions of the tube and the bundle of fibers make it possible to provided a neck on the tube without the corresponding reduction in the inside diameter of the tube preventing the fibers from passing. For example it works well with a tube having outside and inside diameters of 4 mm and 2 mm respectively, and a bundle of fibers having a diameter of about 0.8 mm.
However, if a tube having significantly smaller transverse dimensions is used for a bundle of fibers having the same size, e.g. a tube having an outside diameter of 2.3 mm and an inside diameter of 1.5 mm, thereby reducing the size of the lead-in cables, then these small dimensions do not make it easy to ensure that the block of molded plastic material is held onto the end of the tube. It is no longer possible to make a neck of sufficient depth to hold the block of plastic material securely while retaining an inside diameter that is large enough to allow the bundle of fibers to pass therethrough.
An object of the invention is to provide a longitudinal sealing device which is practical and reliable, in particular for a cable core including a small diameter tube, e.g. a tube having an outside diameter of 2.3 mm and an inside diameter of 1.5 mm.